In recent years, a flexible device in which a functional element such as a semiconductor element, a display element, or a light-emitting element is provided over a substrate having flexibility (hereinafter also referred to as a flexible substrate) has been developed. Typical examples of the flexible device include a lighting device, an image display device, a variety of semiconductor circuits including a semiconductor element such as a transistor, and the like.
As a method for manufacturing a device including a flexible substrate, a technique has been developed in which a functional element such as a thin film transistor or an organic EL element is formed over a formation substrate (e.g., a glass substrate or a quartz substrate), and then the functional element is transferred to a flexible substrate. This technique needs a step of separating a layer including the functional element from the formation substrate (also referred to as a separation step).
For example, Patent Document 1 discloses the following separation technique using laser ablation: a separation layer formed of amorphous silicon or the like is provided over a substrate, a layer to be separated which includes a thin film element is provided over the separation layer, and the layer to be separated is bonded to a transfer body with the use of a bonding layer. The separation layer is ablated by laser light irradiation, so that separation is caused at the separation layer.
In addition, Patent Document 2 discloses a technique in which separation is conducted by physical force with human hands or the like. In addition, Patent Document 2 discloses the following separation technique: a metal layer is formed between a substrate and an oxide layer and separation is caused at the interface between the oxide layer and the metal layer by utilizing weak bonding at the interface between the oxide layer and the metal layer, so that a layer to be separated and the substrate are separated from each other.